This is a continuing study of patients with paroxysmal supraventricular arrhythmia (flutter, fibrillation, and paroxysmal supraventricular tachycardia). Electrophysiological studies are performed for delineating the mechanism of the paroxysmal tachycardia. Drugs are administered, for evaluation of effect on induction of paroxysmal tachycardia. Effective drugs are given (as determined electrophysiologically) and the patients followed in research clinics. The study will attempt to delineate drugs of choice for prevention of recurrent paroxysmal tachycardia.